(a) Field of the Invention
This invention relates to a method and apparatus for carrying a delicate wet paper web on a high speed paper drying machine in the first and second dryer sections.
(b) Description of the Prior Art
On high speed paper drying machines, single felts or fabrics are often used to carry the delicate paper web in the first and second dryer sections. Although the web is fully supported, it can become detached from the felt or fabric at the vacuum wedge as the web adheres to the dryer cylinder, and because of the small vacuum formed as the two surfaces separate. The greater potential problem is at the compression wedge formed between the felt or fabric and the dryer. A large volume of air is carried by the boundary layer on the felt or fabric and this is forced into the nip and pushed through the permeable felt or fabric, to detach the web.
Instabilities in the sheet run in the single-felted paper drying machines are mainly caused by the following mechanisms. Moving surfaces create an under-pressure at the nip of the sheet and the top dryer. This under-pressure creates air flows which tend to lift the sheet off of the felt or fabric. These air flows are directed from the edges of the sheet towards the centre of the sheet. This is one reason for sheet-edge flutter between the top and bottom dryers of the single-felted section.
Moving surfaces create an overpressure at the nip of the felt or fabric and at the bottom dryer. If the felt or fabric is open (i.e., if it has high permeability), the air easily flows through the felt or fabric and lifts the sheet off of the felt or fabric, often across the entire sheet width. If the felt or fabric permeability is low, the air which has entered between the sheet and the felt or fabric at the nip of the sheet and the top dryer cannot escape fast enough through the felt or fabric, and an "air bubble" is created at the nip of the sheet and the top dryer. This bubble is a reason for wrinkles on the sheet.
There are a number of techniques to address this problem, e.g., blasting air out of the compression wedge to create a vacuum or using parallel jets with curved surfaces to generate a negative pressure using the Coanda effect.
One recent technique which was alleged to solve this problem involved blowing air away from the bottom of the nip of the felt or fabric and the bottom dryer and against the boundary layer carried by the felt or fabric at the nip of the felt or fabric and the top dryer. A nozzle in each end of the blow box was alleged to make it possible to maintain a slight vacuum from top to bottom dryer. The vacuum was created by the high velocity air flow from the nozzles based on ejector principles. The result was that the sheet was said to be held steadily to the felt or fabric from both of these nips. This was said to prevent the air build-up between the sheet and the felt or fabric, and to prevent the air bubble from being created at the top nip.